Aid in migration to the new deserialization API

[wprzytula]

Motivation

Let's reduce friction when migrating to the new deserialization API.

What's done

Deprecations

Entities (traits and structs) belonging to the old deserialization API, which we plan to remove soon, are labeled with #[deprecated] attribute. This way they will trigger warnings when used, urging users to consider migration.

Convenient switching between the legacy & the new Session's API

There already are Session and LegacySession. However, a viable migration strategy involves incremental migration from the old API to the new API. In such case, it makes sense to have cheap conversions between Session APIs, using the same data underneath. This is done: LegacySession is convertible to Session and vice versa (and they share the same Cluster through an Arc).

Migration guide

A migration guide is added. It briefly describes the old and the new deserialization traits and compares them. Then, it summarises what changes are needed to adjust to the new deserialization framework entities.

Fixes: #965

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[github-actions]

cargo semver-checks detected some API incompatibilities in this PR.
Checked commit: b7d1dfe

See the following report for details:

cargo semver-checks output

./scripts/semver-checks.sh --baseline-rev 82536814e3341dd2ffdba7366d545474fc0338d7
+ cargo semver-checks -p scylla -p scylla-cql --baseline-rev 82536814e3341dd2ffdba7366d545474fc0338d7
     Cloning 82536814e3341dd2ffdba7366d545474fc0338d7
     Parsing scylla v0.14.0 (current)
      Parsed [  24.970s] (current)
     Parsing scylla v0.14.0 (baseline)
      Parsed [  23.456s] (baseline)
    Checking scylla v0.14.0 -> v0.14.0 (no change)
     Checked [   0.139s] 94 checks: 93 pass, 1 fail, 0 warn, 0 skip

--- failure type_marked_deprecated: #[deprecated] added on type ---

Description:
A type is now #[deprecated]. Downstream crates will get a compiler warning when using this type.
        ref: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-deprecated-attribute
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.36.0/src/lints/type_marked_deprecated.ron

Failed in:
  Struct LegacyQueryResult in /home/runner/work/scylla-rust-driver/scylla-rust-driver/scylla/src/transport/legacy_query_result.rs:52
  Struct LegacyQueryResult in /home/runner/work/scylla-rust-driver/scylla-rust-driver/scylla/src/transport/legacy_query_result.rs:52
  Enum LegacyDeserializationApi in /home/runner/work/scylla-rust-driver/scylla-rust-driver/scylla/src/transport/session.rs:179
  Struct LegacyRowIterator in /home/runner/work/scylla-rust-driver/scylla-rust-driver/scylla/src/transport/iterator.rs:1084
  Struct LegacyTypedRowIterator in /home/runner/work/scylla-rust-driver/scylla-rust-driver/scylla/src/transport/iterator.rs:1137

     Summary semver requires new minor version: 0 major and 1 minor checks failed
    Finished [  48.624s] scylla
     Parsing scylla-cql v0.3.0 (current)
      Parsed [  11.259s] (current)
     Parsing scylla-cql v0.3.0 (baseline)
      Parsed [  11.304s] (baseline)
    Checking scylla-cql v0.3.0 -> v0.3.0 (no change)
     Checked [   0.114s] 94 checks: 94 pass, 0 skip
     Summary no semver update required
    Finished [  22.726s] scylla-cql
make: *** [Makefile:61: semver-rev] Error 1

[wprzytula]

Rebased on main after #1117 was merged.