Update logic for fetching CF entities used in proxy headers

[markdboyd]

Changes proposed in this pull request:

Related to https://github.com/cloud-gov/private/issues/2032

• Update logic for fetching CF entities (org, spaces) used as proxy headers to ensure that only unique values are returned. The CF API endpoints may return the same org or space GUID in the results, which is redundant, but also causes the headers using these values to be larger than necessary
• Add unit tests for updated CF entity fetching logic

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

Nothing about the mechanics of how the proxy passes information to OpenSearch via headers has changed, we have just fixed a bug that was causing duplicate information to be sent.

[JasonTheMain]

Makes sense to convert them to sets