Improve client init time by switching to regex-lite (#3269)

Each client initialization was taking between 1 and 2 milliseconds,
regardless if the client had been constructed before or not. For
example, if a customer wants five clients with different credentials
providers, that could be 10 milliseconds of time spent in
`Client::from_conf`. Approximately 98% of this time was spent compiling
regular expressions for the endpoint partition resolver.

This change switches everything over to the regex-lite crate, which has
faster regex compile times, and shouldn't have much of an impact on
performance for our specific use-cases (small strings, only evaluated at
client initialization).

The use of regex was entirely removed in aws-sigv4 since it was overkill
for what it was being used for.

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