Draft: Add PoC endpoint for unified AI Gateway architecture

This MR adds a new internal endpoint that can be used to make parameterised code suggestion requests.

The new endpoint can be called under

POST /internal/v2/code_completions

with the following payload

Attribute	Type	Required	Description	Example
prompt_version	int	no	The version of the prompt	1
project_path	string	no	The name of the project (max_len: 255)	gitlab-orb/gitlab-shell
project_id	int	no	The id of the project (max_len: 255)	33191677
current_file.file_name	string	yes	The name of the current file (max_len: 255)	README.md
current_file.content_above_cursor	string	yes	The content above cursor (max_len: 100,000)	import numpy as np
current_file.content_below_cursor	string	yes	The content below cursor (max_len: 100,000)	def __main__:\n
model	string	no	The desired model provider (text-bison/code-bison/code-gecko)	code-gecko
parameters.temperature	float	no		0.2
parameters.max_decode_steps	int	no		16
parameters.top_p	float	no		0.95
parameters.top_k	int	no		40
instances	dict[string, string]	no		{"prefix": "some prefix for the model"}

Current limitations:

• maxOutputTokens not implemented
• Parameters and instances only sent to 3rd party models
• Some edge cases are not covered
• Quick and dirty fix

Relates to #163 (closed)