Shared reqwest client between fetch calls

[lucacasonato]

Maybe fixes #6696

[ry]

LGTM - that was easier than I expected it to be. Nice work.

[rudoi]

Curious, does this complicate #3444 at all? Should the presence of additional arguments signal that we should not use the global client?

[Spoonbender]

@rudoi in connection pools implementations I've seen, the pool is keyed in a way which represents all relevant input properties of the connection. In other words, the additional arguments shouldn't cause Deno to use a non-pooled connection - instead, there should be a pool for each unique permutation of the arguments that is being used. In the DB connection pooling world it is fairly easy: the key is the connection string (after some normalization). In Deno's HTTP client pooling, we can perhaps serialize the input arguments to JSON and use that for keying.