Add E2EE example, style changes, more tests

[skheyfets-asapp]

• Addressed code style comments
• Switched to require testifiy package in encryption tests
• Refactored and added encryption utilities with ability to reuse AES cipher
• Added echo example with E2EE support (audio decrypted and reencrypted before sending back)
• Added benchmark tests for encryption utilities (results from dev machine below)

goos: darwin
goarch: amd64
pkg: github.com/livekit/server-sdk-go/v2
cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
BenchmarkDecryptAudioNewCipher-12                1781293               603.1 ns/op          1104 B/op          6 allocs/op
BenchmarkDecryptAudioCachedCipher-12             2778477               429.1 ns/op           592 B/op          5 allocs/op
BenchmarkEncryptAudioCachedCipher-12             1287099               884.3 ns/op           632 B/op          6 allocs/op
BenchmarkEncryptAudioNewCipher-12                1080866              1035 ns/op            1144 B/op          7 allocs/op

[boks1971]

Thank you. Guess not a huge performance win, but a sizeable one!

[boks1971]

Thank you for doing this @skheyfets-asapp . I will merge this now.