Fix a potential race: StaticOIDCClientsStore concurrent access

[ErmakovDmitriy]

This PR introduces an RWMutex to protect StaticOIDCClientsStore access.

I read the StaticOIDCClientsStore implementation and think that a race condition is possible under some circumstances, in particular when a new OIDCClientConfig is added (just after the auth agent startup) and at the same time HAProxy causes the auth agent to lookup for another OIDCClientConfig for another domain.

I tried to write a test case (in the commit) to check how it behaved and before I added the sync.RWMutex, I managed to get, sometimes (once every 5-8 test runs) an error:

fatal error: concurrent map read and map write

goroutine 9 [running]:
github.com/criteo/haproxy-spoe-auth/internal/auth.(*StaticOIDCClientsStore).GetClient(...)
        /home/demonihin/projects/haproxy-spoe-auth/internal/auth/oidc_clients_store.go:38
github.com/criteo/haproxy-spoe-auth/internal/auth.TestStaticOIDCClientsStoreRace.func2()
        /home/demonihin/projects/haproxy-spoe-auth/internal/auth/oidc_clients_store_test.go:38 +0x78
created by github.com/criteo/haproxy-spoe-auth/internal/auth.TestStaticOIDCClientsStoreRace in goroutine 6
        /home/demonihin/projects/haproxy-spoe-auth/internal/auth/oidc_clients_store_test.go:36 +0x114

goroutine 1 [runnable]:
internal/poll.(*FD).Write(0x400014bc18, {0x222380, 0x400000e348, 0x100004000103450})
        /opt/go/src/internal/poll/fd_unix.go:366
os.(*File).write(...)
        /opt/go/src/os/file_posix.go:46
os.(*File).Write(0x4000052030, {0x40000108b8?, 0x5, 0xf202c?})
        /opt/go/src/os/file.go:183 +0x5c
fmt.Fprint({0x293048, 0x4000052030}, {0x400014bdb8, 0x2, 0x2})
        /opt/go/src/fmt/print.go:263 +0x74
fmt.Print(...)
        /opt/go/src/fmt/print.go:272
testing.(*M).Run(0x4000104dc0)
        /opt/go/src/testing/testing.go:1961 +0x8bc
main.main()
        _testmain.go:59 +0x1a8

goroutine 7 [runnable]:
strings.Clone(...)
        /opt/go/src/strings/clone.go:25
github.com/criteo/haproxy-spoe-auth/internal/auth.(*StaticOIDCClientsStore).AddClient(0x400006e2e0, {0x400004f790, 0xe}, {0x22d5fe, 0x9}, {0x22eab3, 0xd}, {0x4000200340, 0x1f})
        /home/demonihin/projects/haproxy-spoe-auth/internal/auth/oidc_clients_store.go:49 +0xac
github.com/criteo/haproxy-spoe-auth/internal/auth.TestStaticOIDCClientsStoreRace.func1()
        /home/demonihin/projects/haproxy-spoe-auth/internal/auth/oidc_clients_store_test.go:26 +0xa4
created by github.com/criteo/haproxy-spoe-auth/internal/auth.TestStaticOIDCClientsStoreRace in goroutine 6
        /home/demonihin/projects/haproxy-spoe-auth/internal/auth/oidc_clients_store_test.go:17 +0xec
exit status 2
FAIL    github.com/criteo/haproxy-spoe-auth/internal/auth       0.040s