feat(STX-433): add submissionMethod to transaction metadata

[rarquevaux]

Explanation

The goal of this PR is to add a submissionMethod property in the TransactionMetadata object in order to record how the transaction was submitted to the network and add it to the metrics. This datapoint can be recording at submission in the https://github.com/MetaMask/metamask-extension/blob/main/app/scripts/controller-init/confirmations/transaction-controller-init.ts

Draft PR on extension to demonstrate the use case: MetaMask/metamask-extension#41465

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Low Risk
Low risk type-only change: adds an optional metadata field and enum export, with no runtime behavior changes, but may require downstream projects to update typings/imports.

Overview
Adds an optional submissionMethod field to TransactionMeta to record how a transaction was submitted, backed by a new TransactionSubmissionMethod enum (currently sentinel_stx and sentinel_relay).

Exports TransactionSubmissionMethod from the package entrypoint and documents the new metadata field in the transaction-controller changelog.

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