fix(aws): Get region from IMDS when missing for awssmfs, awssmpfs, and blobfs (s3://)

[hairyhenderson]

In the original v2 aws-sdk-go, it wasn't necessary to set an AWS region, but with v2, a region is always required (at least, for most APIs). This can either be done with environment variables (AWS_REGION, for example), or by a shared config. On EC2, however, it's customary to fall back to the instance metadata service (IMDS).

This is necessary for hairyhenderson/gomplate#2180, for example.

[github-actions]

Benchmark

Benchmark suite	Current: d01b2d1	Previous: 6408c34	Ratio
BenchmarkPathForDirFS	45.47 ns/op 0 B/op 0 allocs/op	46.77 ns/op 0 B/op 0 allocs/op	0.97
BenchmarkPathForDirFS - ns/op	45.47 ns/op	46.77 ns/op	0.97
BenchmarkPathForDirFS - B/op	0 B/op	0 B/op	1
BenchmarkPathForDirFS - allocs/op	0 allocs/op	0 allocs/op	1
BenchmarkSplitRepoPath/0	12.09 ns/op 0 B/op 0 allocs/op	12.09 ns/op 0 B/op 0 allocs/op	1
BenchmarkSplitRepoPath/0 - ns/op	12.09 ns/op	12.09 ns/op	1
BenchmarkSplitRepoPath/0 - B/op	0 B/op	0 B/op	1
BenchmarkSplitRepoPath/0 - allocs/op	0 allocs/op	0 allocs/op	1
BenchmarkSplitRepoPath/1	16.74 ns/op 0 B/op 0 allocs/op	16.87 ns/op 0 B/op 0 allocs/op	0.99
BenchmarkSplitRepoPath/1 - ns/op	16.74 ns/op	16.87 ns/op	0.99
BenchmarkSplitRepoPath/1 - B/op	0 B/op	0 B/op	1
BenchmarkSplitRepoPath/1 - allocs/op	0 allocs/op	0 allocs/op	1
BenchmarkSplitRepoPath/2	40.04 ns/op 4 B/op 1 allocs/op	53.2 ns/op 4 B/op 1 allocs/op	0.75
BenchmarkSplitRepoPath/2 - ns/op	40.04 ns/op	53.2 ns/op	0.75
BenchmarkSplitRepoPath/2 - B/op	4 B/op	4 B/op	1
BenchmarkSplitRepoPath/2 - allocs/op	1 allocs/op	1 allocs/op	1
BenchmarkSplitRepoPath/3	40.73 ns/op 8 B/op 1 allocs/op	53.95 ns/op 8 B/op 1 allocs/op	0.75
BenchmarkSplitRepoPath/3 - ns/op	40.73 ns/op	53.95 ns/op	0.75
BenchmarkSplitRepoPath/3 - B/op	8 B/op	8 B/op	1
BenchmarkSplitRepoPath/3 - allocs/op	1 allocs/op	1 allocs/op	1
BenchmarkSplitRepoPath/4	38.46 ns/op 8 B/op 1 allocs/op	47.32 ns/op 8 B/op 1 allocs/op	0.81
BenchmarkSplitRepoPath/4 - ns/op	38.46 ns/op	47.32 ns/op	0.81
BenchmarkSplitRepoPath/4 - B/op	8 B/op	8 B/op	1
BenchmarkSplitRepoPath/4 - allocs/op	1 allocs/op	1 allocs/op	1
BenchmarkSplitRepoPath/5	44.19 ns/op 4 B/op 1 allocs/op	54.29 ns/op 4 B/op 1 allocs/op	0.81
BenchmarkSplitRepoPath/5 - ns/op	44.19 ns/op	54.29 ns/op	0.81
BenchmarkSplitRepoPath/5 - B/op	4 B/op	4 B/op	1
BenchmarkSplitRepoPath/5 - allocs/op	1 allocs/op	1 allocs/op	1
BenchmarkSplitRepoPath/6	16.75 ns/op 0 B/op 0 allocs/op	17.23 ns/op 0 B/op 0 allocs/op	0.97
BenchmarkSplitRepoPath/6 - ns/op	16.75 ns/op	17.23 ns/op	0.97
BenchmarkSplitRepoPath/6 - B/op	0 B/op	0 B/op	1
BenchmarkSplitRepoPath/6 - allocs/op	0 allocs/op	0 allocs/op	1

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