feat(rust): Add tags to AssumeRoleProviderBuilder

[c-thiel]

Motivation and Context

See awslabs/aws-sdk-rust#1366

Description

See awslabs/aws-sdk-rust#1366

Checklist

• For changes to the AWS SDK, generated SDK code, or SDK runtime crates, I have created a changelog entry Markdown file in the .changelog directory, specifying "aws-sdk-rust" in the applies_to key.

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[c-thiel]

Not really sure about the changelog. Please change it as desired.

[rcoh]

thanks!

[ysaito1001]

Just realized one thing. This CI check complains that the tag method now exposes a type that previously wasn't exposed. However, the method cannot take the Tag type from aws_sdk_sts directly either because aws-config should only depend on so-called runtime crates publicly (those listed in its allowed external-types) but not on generated SDK crates like aws-sdk-sts (ok to depend on them privately as implementation details).

One way to work around this is for the tag method to take plain Strings for key & value for tags, like

pub fn tags<K, V>(mut self, tags: impl IntoIterator<Item = (K, V)>) -> Self 
where
    K: Into<String>,
    V: Into<String>,
{...}

This is why a sibling method policy_arns takes a vec of plain Strings instead of a vec of PolicyDescriptorTypes.

Alternatively, we could define own tag type somewhere in the aws-config crate, and let the tags method take a vec of that tag type instead.

[c-thiel]

Just realized one thing. This CI check complains that the tag method now exposes a type that previously wasn't exposed. However, the method cannot take the Tag type from aws_sdk_sts directly either because aws-config should only depend on so-called runtime crates publicly (those listed in its allowed external-types) but not on generated SDK crates like aws-sdk-sts (ok to depend on them privately as implementation details).

One way to work around this is for the tag method to take plain Strings for key & value for tags, like

pub fn tags<K, V>(mut self, tags: impl IntoIterator<Item = (K, V)>) -> Self 
where
    K: Into<String>,
    V: Into<String>,
{...}

This is why a sibling method policy_arns takes a vec of plain Strings instead of a vec of PolicyDescriptorTypes.

Alternatively, we could define own tag type somewhere in the aws-config crate, and let the tags method take a vec of that tag type instead.

@ysaito1001

I made the change, but we have a small problem here: Due to the absolutely awful builder of Tag, there is not typesafe constructor for this dead-simple type. The build method only fails if either key or value is None, which in our implementation cannot be the case. Hence I currently use unwrap, even though this is not nice.
The alternative would be to expose the Error type, this is however a type the sts crate again.