Add multiline support for secrets

[Arminmsg]

Desired Outcome

Currently the action just accepts single line values for the secret section. If you need to access multiple secrets this is difficult to read and maintain.
I'm introducing a multiline capability so it's easier to maintain multiple secrets.

Implemented Changes

Describe how the desired outcome above has been achieved with this PR. In
particular, consider:
I've changed the parsing of the INPUT_SECRET inside the entrypoint.sh. If a multiline string is detected the new lines are removed, so it's continued to be parsed as a single line string.

Connected Issue/Story

Resolves #[/issues/12]

Definition of Done

At least 1 todo must be completed in the sections below for the PR to be
merged.

Changelog

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a
  CHANGELOG update

Test coverage

• This PR includes new unit and integration tests to go with the code
  changes, or
• The changes in this PR do not require tests

Documentation

• Docs (e.g. READMEs) were updated in this PR
• A follow-up issue to update official docs has been filed here: [insert issue ID]
• This PR does not require updating any documentation

Behavior

• This PR changes product behavior and has been reviewed by a PO, or
• These changes are part of a larger initiative that will be reviewed later, or
• No behavior was changed with this PR

Security

• Security architect has reviewed the changes in this PR,
• These changes are part of a larger initiative with a separate security review, or
• There are no security aspects to these changes

[davidh-cyberark]

change looks good to me. I would like to see at least 1 test case pass before merging.