FPO-419: Adds automated model build workflow

[willfish]

Jira link

FPO-419

What?

I have added/removed/altered:

• Added a workflow to automate releasing a new model on the 1st and 15th of each month
• Added a script to create the PR and bump the version
• Added a script to install gh cli since there's no official orb to do this
• When validation of the model fails, make sure to close the PR, delete the branch and exit
• When the validation succeeds and after post-deployment tests merge the PR to enable automated production deployments

Why?

I am doing this because:

• This enables us to continually keep search references, commodities, etc, dynamically updated

AC

• Models are successfully being generated on a schedule
• Automated PRs are closed off (either merged/closed and deleted) when we know the result

[amberstarlight]

Getting this running in CI and it began running in an infinite loop 🥴 debugging the CircleCi workflow.

[amberstarlight]

Re-adding the job triggers resolved that. However there's some Python errors (looks unrelated to this PR) that I want to clear out before merging.

[amberstarlight]

The import error (from cached_download, from sentence-transformers) is resolved when bumping sentence-transformers upwards of v2.3.0 - see below issue.

UKPLab/sentence-transformers#3001 (comment)