Add GraphQL Mutation to Create Credentials for Host

[jabbate19]

What type of PR is this?

/kind feature
/kind api-change

What this PR does / why we need it:

If we have external tools that can provide us credentials, there is currently no easy way for those to be inserted.

• Credentials no longer require a Task.
• Credentials can be created via mutation

Which issue(s) this PR fixes:

[hulto]

Do you need to:

• Implement generated the generated mutation resolver method in tavern/internal/graphql/mutation.resolvers.go ?
• write a unit test for your new mutation by defining YAML test cases in a new

[jabbate19]

Do you need to:

• Implement generated the generated mutation resolver method in tavern/internal/graphql/mutation.resolvers.go ?
• write a unit test for your new mutation by defining YAML test cases in a new

@hulto Fixed!