OPDATA-3889: Return a specific result from FTSE parsers

[dskloetc]

https://smartcontract-it.atlassian.net/browse/OPDATA-3889

Description

The FTSE parsers return records with multiple values for each of the index types.
But the EA should return one of those numbers as the main result.
The right place to determine which piece of the record should be the main result is in the individual parsers.

So in this PR we change the interface of the parsers to include one number as result and the whole record of values as parsedData.

Changes

1. Change parsers interface.
2. In FTSE parser, return gbpIndex as the result.
3. In Russell parsers, return close as the result.
4. Drive-by fix: Change the type of numberOfConstituents and gbpIndex from number | null to just number as there is no way for them to be null.

Steps to Test

1. Unit tests updated.
2. Tested end-to-end in another branch.

Quality Assurance

• If a new adapter was made, or an existing one was modified so that its environment variables have changed, update the relevant infra-k8s configuration file.
• If a new adapter was made, or an existing one was modified so that its environment variables have changed, update the relevant adapter-secrets configuration file or update the soak testing blacklist.
• If a new adapter was made, or a new endpoint was added, update the test-payload.json file with relevant requests.
• The branch naming follows git flow (feature/x, chore/x, release/x, hotfix/x, fix/x) or is created from Jira.
• This is related to a maximum of one Jira story or GitHub issue.
• Types are safe (avoid TypeScript/TSLint features like any and disable, instead use more specific types).
• All code changes have 100% unit and integration test coverage. If testing is not applicable or too difficult to justify doing, the reasoning should be documented explicitly in the PR.

[changeset-bot]

🦋 Changeset detected

Latest commit: a1b3508

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@chainlink/ftse-sftp-adapter	Patch

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