refactor: Make OutgoingWebhookServiceInterface more interface like.
refactor: Make OutgoingWebhookServiceInterface more interface like.

Using the Python Standard Library's abc library and NotImplementedError
we can better define interfaces (this is mainly to improve readability
and consistency).

Signed-off-by: Hemanth V. Alluri <1aa36b69b223107bb38f42dc04c4ff72441acfa6@gmail.com>

shubhamdhama/zulip,timabbott/zulip,rht/zulip,brainwane/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip,synicalsyntax/zulip,brainwane/zulip,andersk/zulip,timabbott/zulip,showell/zulip,punchagan/zulip,timabbott/zulip,andersk/zulip,brainwane/zulip,brainwane/zulip,synicalsyntax/zulip,zulip/zulip,hackerkid/zulip,showell/zulip,showell/zulip,punchagan/zulip,zulip/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,brainwane/zulip,kou/zulip,zulip/zulip,timabbott/zulip,showell/zulip,shubhamdhama/zulip,timabbott/zulip,kou/zulip,timabbott/zulip,rht/zulip,rht/zulip,synicalsyntax/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,rht/zulip,andersk/zulip,hackerkid/zulip,kou/zulip,hackerkid/zulip,hackerkid/zulip,shubhamdhama/zulip,zulip/zulip,synicalsyntax/zulip,synicalsyntax/zulip,synicalsyntax/zulip,kou/zulip,showell/zulip,punchagan/zulip,showell/zulip,synicalsyntax/zulip,punchagan/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,kou/zulip,andersk/zulip,brainwane/zulip,shubhamdhama/zulip,timabbott/zulip,shubhamdhama/zulip,shubhamdhama/zulip,brainwane/zulip,showell/zulip,shubhamdhama/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,kou/zulip