Vaaranan yaser annie/create volunteer signup implementation and interface

[vaaranan-y]

Notion ticket link

Ticket Name

Implementation description

• Created skeleton code for interface and implementation of VolunteerSignUp Services (i.e. getVolunteerSignup, postVolunteerSignup, editVolunteerSignup)
• Return types, implementation code, and parameter types to be added in next ticket

Steps to test

1. Not really testable, however verification of function names and file definitions would be appreciated!

What should reviewers focus on?

• Simply the definitions of the two files created for this ticket (do they match the standards of services we want to implement in this project)

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• I have run the appropriate linter(s)
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR