feat: Expose ConnectionPool customization on FlagsmithConfig.Builder

[adamvialpando]

Summary

Adds connectionPool(ConnectionPool) to FlagsmithConfig.Builder so users can tune OkHttp's idle connection behavior (keepAlive duration and max idle connections). This is the API requested in #207 for users running behind intermediaries (AWS ALB, nginx) that close idle sockets sooner than OkHttp's default of 5 minutes.

Default behavior is unchanged: when connectionPool() is not called (or is called with null), OkHttp's default pool is used, so existing users see no difference.

Usage

FlagsmithConfig config = FlagsmithConfig.newBuilder()
    .connectionPool(new ConnectionPool(5, 30, TimeUnit.SECONDS))
    .build();

Tests

Two new tests in FlagsmithConfigTest:

1. configTest_customConnectionPool_respectsKeepAliveAndMaxIdle reflects into ConnectionPool.delegate and asserts the user's maxIdleConnections and keepAliveDurationNs reach the live pool. Mutation tested: removing the constructor wiring fails this test.
2. configTest_nullConnectionPool_isSafeAndUsesDefault passes null explicitly and asserts no NPE; a default OkHttp pool is still wired. Mutation tested: removing the null guard fails this test.

Both tests pass on OkHttp 5.x and OkHttp 4.x (via the test-okhttp4 profile).

[emyller]

Looks great!