Add build warnings to `ExecutionData`

[Enkidu93]

SF intends to expose the execution data field to Serval admins (EITL). They've requested that we expose warnings there related to the build. From SF:

Requirements

Add a warnings property to the build.executionData:

[
  "The draft quality may be bad because only 113 verses were included in the training data, and the target languge code, abc is not natively supported by the model.",
  "Versification problems",
  "Tokenization problems"
]

Rationale

• We don’t currently have a major need to show these to end users, so making them parsable/localizable isn’t currently a priority
• It easily allows adding more warnings in the future, with no changes to the API
• If/when we decide to show them to end users, a particular warning can have a specific API defined

Copied from this document: https://docs.google.com/document/d/1kLb3Sx4c7QGq-gs7n17yeAnRq5wx1dWeKaht8oUG_b0/edit?tab=t.0#heading=h.tt1gmrpe8lgb.

Related to #755, sillsdev/machine#331

[Enkidu93]

The initial warnings will include:

• Versification/USFM mismatch warnings
• A warning if the script is unknown to NLLB
• A warning if there are fewer than 500 segments of training data

[Enkidu93]

Added here #808