Add a debug log entry for retries

[jdisanti]

Motivation and Context

This PR logs a debug event when a retry is going to be performed so that it's easier to tell retry is working by looking at logs (prompted by awslabs/aws-sdk-rust#524).

Checklist

• I have updated CHANGELOG.next.toml if I made changes to the smithy-rs codegen or runtime crates
• I have updated CHANGELOG.next.toml if I made changes to the AWS SDK, generated SDK code, or SDK runtime crates

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Server Test

A new doc preview is ready to view.

Rust Wrk benchmark report:

Duration: 90 sec, Connections: 32, Threads: 2

Measurement	Deviation	Current	Old
Requests/sec	-8.05%	60290.99	65570.48
Total requests	-8.05%	5430219	5905413
Total errors	NaN%	0	0
Total successes	-8.05%	5430219	5905413
Average latency ms	-11.59%	0.61	0.69
Minimum latency ms	0.00%	0.02	0.02
Maximum latency ms	16.40%	21.22	18.23
Stdev latency ms	-28.16%	0.74	1.03
Transfer Mb	-8.05%	564.47	613.87
Connect errors	NaN%	0	0
Read errors	NaN%	0	0
Write errors	NaN%	0	0
Status errors (not 2xx/3xx)	NaN%	0	0
Timeout errors	NaN%	0	0

[rcoh]

LGTM, but seems like we might as well add a little more info?

[rcoh]

should we include more info here? retry kind, number of retries, etc.?

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Server Test

A new doc preview is ready to view.

Rust Wrk benchmark report:

Duration: 90 sec, Connections: 32, Threads: 2

Measurement	Deviation	Current	Old
Requests/sec	16.70%	72005.71	61698.93
Total requests	16.72%	6482332	5553748
Total errors	NaN%	0	0
Total successes	16.72%	6482332	5553748
Average latency ms	-3.51%	0.55	0.57
Minimum latency ms	0.00%	0.02	0.02
Maximum latency ms	-1.33%	17.04	17.27
Stdev latency ms	36.67%	0.82	0.6
Transfer Mb	16.72%	673.84	577.31
Connect errors	NaN%	0	0
Read errors	NaN%	0	0
Write errors	NaN%	0	0
Status errors (not 2xx/3xx)	NaN%	0	0
Timeout errors	NaN%	0	0