fix(admin): allow to select only existing extSource when adding identity

Description

fix(admin): allow to select only existing extSource when adding identity

• when adding identity the user could search for an extSource and select it, but also could just send any custom search text as the extSource
• This then sent a string instead of extSource object to the backend
• Changed the search component to entity search select so an extSource always has to be selected

How to test

user -> identities -> add identity

Author's checklist

• I have followed the contribution guidelines
• This MR has been tested or does not change functionality
• I have added relevant merge request dependencies (if this MR has any)
• I have added the correct labels
• I have assigned reviewers (if any are relevant)
• I have edited the documentation (if the changes require it) or I have noted the need for the change if I do not have access to the documentation
• I have marked all introduced BREAKING CHANGES or necessary DEPLOYMENT NOTES in the commit message(s)

Reviewer's checklist

• This MR has been tested or does not change functionality
• This MR has correct commit message format

Related issues

Closes STR-1210