C2-2727: improve loadAuthenticationSubject performance

[ivansenic]

What this PR does:
Improves perf of the authentication user serialization, as it appears on the hot path with 5% alloc samples:

• remove the builtinContainerAttributes for immutable classes
• avoid using builders
• better map construction

Micro-benchmarking shows 2x throughput and 3x less allocation per op:

Before

 * Benchmark                                                                       (propertyCount)   Mode  Cnt     Score    Error   Units
 * AuthenticationSerializationBench.loadAuthenticationSubject                                    2  thrpt    5     1.642 ±  0.044  ops/us
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.alloc.rate                     2  thrpt    5  2630.386 ± 70.494  MB/sec
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.alloc.rate.norm                2  thrpt    5  1680.000 ±  0.001    B/op
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.count                          2  thrpt    5   333.000           counts
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.time                           2  thrpt    5   223.000               ms
 * AuthenticationSerializationBench.loadAuthenticationSubject                                    4  thrpt    5     1.103 ±  0.045  ops/us
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.alloc.rate                     4  thrpt    5  2440.713 ± 99.410  MB/sec
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.alloc.rate.norm                4  thrpt    5  2320.000 ±  0.001    B/op
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.count                          4  thrpt    5   338.000           counts
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.time                           4  thrpt    5   228.000               ms

After

 * Benchmark                                                                       (propertyCount)   Mode  Cnt     Score     Error   Units
 * AuthenticationSerializationBench.loadAuthenticationSubject                                    2  thrpt    5     3.384 ±   0.230  ops/us
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.alloc.rate                     2  thrpt    5  1807.395 ± 123.001  MB/sec
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.alloc.rate.norm                2  thrpt    5   560.000 ±   0.001    B/op
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.count                          2  thrpt    5   338.000            counts
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.time                           2  thrpt    5   228.000                ms
 * AuthenticationSerializationBench.loadAuthenticationSubject                                    4  thrpt    5     2.137 ±   0.029  ops/us
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.alloc.rate                     4  thrpt    5  1728.442 ±  23.497  MB/sec
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.alloc.rate.norm                4  thrpt    5   848.000 ±   0.001    B/op
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.count                          4  thrpt    5   333.000            counts
 * AuthenticationSerializationBench.loadAuthenticationSubject:·gc.time                           4  thrpt    5   227.000                ms

Which issue(s) this PR fixes:
Internal issue.

Tasks:

• add readme for the microbench module

[maheshrajamani]

LGTM