[services] Add attachment delimiter

Summary:
Depends on D4075

Adding an attachment holders delimiter. Holders are built from:

• hashes - these are hexadecimal values - 0 to 9 and A to F (or lower case)
• uuid - also hexadecimal - 0 to 9 and A to F (or lower case)
• ID_SEPARATOR - :

So it's impossible to have ; character that would not be a delimiter.

Test Plan: Services still build

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4076