Update createLetter lob function to appnd with merge variables

[nancy-luu]

• User input is passed as merge variable {{constituent_input}}

• If merge variable exists, mergeVariable object is set for lob letter creation

• Ensured merge variable payloads are within lob's constraints:

1. It can be no longer than 25,000 characters.
2. Variable names cannot contain any whitespace or any of the following special characters: !, ", #, %, &, ', (, ), *, +, ,, /, ;, <, =, >, @, [, , ], ^, `, {, |, }, ~

• Constituant input is appended to template_id

[github-actions[bot]]

[github-actions[bot]]

[github-actions[bot]]

Hello there, thanks for opening your first Pull Request. Someone will review it soon.