Fix clippy warning in common when using only fs flag

[Alenar]

Content

This PR fix the clippy warning that can be seen in some CI jobs :

Run cargo build --lib --release --package mithril-stm --package mithril-client --features full,unstable 
   Compiling mithril-common v0.3.2+80ee079 (/home/runner/work/mithril/mithril/mithril-common)
   Compiling mithril-stm v0.3.15+80ee079 (/home/runner/work/mithril/mithril/mithril-stm)
warning: function `try_inspect` is never used
  --> mithril-common/src/chain_observer/model.rs:33:12
   |
33 |     pub fn try_inspect<R>(inner: Vec<u8>) -> StdResult<R>
   |            ^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: type alias `Datums` is never used
  --> mithril-common/src/chain_observer/model.rs:46:14
   |
46 |     pub type Datums = Vec<TxDatum>;
   |              ^^^^^^

warning: `mithril-common` (lib) generated 2 warnings
   Compiling mithril-client v0.6.2+80ee079 (/home/runner/work/mithril/mithril/mithril-client)
    Finished release [optimized] target(s) in 14.12s

It also add a cfg_fs_random "flag" in mithril-common to correctly document the feature flag needed by some objects.

Pre-submit checklist

• Branch
  • Crates versions are updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • No clippy warnings in the CI
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested

[dlachaume]

LGTM 🚀

[github-actions]

Test Results

  3 files  ±0   40 suites  ±0   9m 4s ⏱️ -17s
830 tests ±0  830 ✅ ±0  0 💤 ±0  0 ❌ ±0 
917 runs  ±0  917 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 264bd59. ± Comparison against base commit 939e16e.

♻️ This comment has been updated with latest results.

[jpraynaud]

LGTM 👍