feat(common): set default StreamableFile length automatically when possible

[jonahsnider]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Users must always explicitly provide options.length when constructing a StreamableFile.

What is the new behavior?

When a Uint8Array is provided to the constructor of StreamableFile, options.length will be ??= to the Uint8Array's length.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 98560942-593e-45a6-bccb-de3ce381fc12

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 94.121%

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Totals
Change from base Build 4207ecc6-d73b-40a7-9a5d-57e04fd83620:	0.001%
Covered Lines:	5763
Relevant Lines:	6123

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💛 - Coveralls