ccl/sqlproxyccl: add --disable-connection-rebalancing flag to "mt start-proxy"

[jaylim-crl]

Previously, we added the connection rebalancing feature to the proxy, and that
gets enabled automatically. This feature will not work with < v22.1 clusters
since it relies on the session migration work that was added recently. In
theory, the proxy will still work, but the balancer will constantly attempt
to rebalance connection when we know that it will fail, and this adds to the
overall latency of the connection since we suspend the processors during
connection migration.

To allow us to transition to v22.1 nicely, we will introduce a new flag
--disable-connection-rebalancing to the mt start-proxy subcommand. When
that flag is set, all connection rebalancing operations will be disabled. We
have to roll out sqlproxy to CC before the clusters get their major upgrade
since it contains some auth work that is a pre-req to v22.1. To avoid these
latency issues, sqlproxy will be rolled out with the flag set. Once clusters
have been upgraded to v22.1, the flag will be removed during startup.

No release notes since mt start-proxy is internal only.

Release note: None

[cockroach-teamcity]

This change is 

[jaylim-crl]

Strange. Couldn't seem to get the bazel tests to pass, but they all pass locally:

INFO: Build completed successfully, 401 total actions
//pkg/ccl/sqlproxyccl:sqlproxyccl_test                          (cached) PASSED in 45.2s
//pkg/ccl/sqlproxyccl/balancer:balancer_test                             PASSED in 5.9s
//pkg/ccl/sqlproxyccl/denylist:denylist_test                             PASSED in 1.8s
//pkg/ccl/sqlproxyccl/interceptor:interceptor_test                       PASSED in 0.6s
//pkg/ccl/sqlproxyccl/tenant:tenant_test                                 PASSED in 10.9s
//pkg/ccl/sqlproxyccl/throttler:throttler_test                           PASSED in 0.3s

[jaylim-crl]

Making this a draft to figure out what went wrong with the bazel tests.

[jaylim-crl]

It turned out that we had to bump the bazel test size from small to medium because we're hitting our timeout threshold. Hopefully tests are green this time.

[jeffswenson]

LGTM

[jaylim-crl]

Flaked on pkg/sql/logictest, which is unrelated to this PR.

bors r=JeffSwenson

[jaylim-crl]

TFTR!

[craig]

Build succeeded:

• GitHub CI (Cockroach)