chore: Add regex validation for session name to relevant resolvers

[sichanyoo]

Issue #

• Add regex validation for resolvers that use session name #1695

Description of changes

• Add regex validation for session name to STSAssumeRole and STSWebIdentity credential resolver initializers so that error gets caught and thrown wayyyy before it does without this change (i.e., bad request exception returned from STS server)

New/existing dependencies impact assessment, if applicable

Conventional Commits

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jbelkins]

See my comment, you're actually using Objective-C regex support.

Also fix the swiftlint warning

[jbelkins]

After reviewing docs, Swift regex support requires macOS platform support above our minimum version.

Approving with Foundation regex support as written.