Migrate to structured logging using "go.uber.org/zap".

[fancycode]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 2103082377

• 122 of 541 (22.55%) changed or added relevant lines in 22 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 43.496%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
natsclient_loopback.go	3	4	75.0%
proxy/proxy_client.go	0	3	0.0%
virtualsession.go	2	5	40.0%
natsclient.go	7	11	63.64%
proxy/proxy_session.go	0	5	0.0%
proxy/proxy_tokens_static.go	5	10	50.0%
backend_configuration.go	14	22	63.64%
logger.go	9	19	47.37%
proxy/proxy_tokens_etcd.go	0	11	0.0%
janus_client.go	0	13	0.0%

Files with Coverage Reduction	New Missed Lines	%
natsclient.go	1	51.81%

Totals
Change from base Build 2101125762:	0.04%
Covered Lines:	4350
Relevant Lines:	10001

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💛 - Coveralls

[fancycode]

Go supports native structured logging through the slog package since 1.21, so this should be used instead of go.uber.org/zap.

For 1.20, the https://pkg.go.dev/golang.org/x/exp/slog could be used.