Add endpoint to update molecular mechanism

[dglemos]

Ticket: G2P-182
We only allow updates if the mechanism is 'undetermined'.

New endpoint
gene2phenotype/api/lgd/<str:stable_id>/update_mechanism/

The input data has the same format as the mechanism data sent by the curation page.

To test
db: dlemos_G2P_2024_18
Input data:

{
    "molecular_mechanism": {
				"name": "gain of function",
				"support": "evidence"
		    },
    "mechanism_synopsis": {
				"name": "",
				"support": ""
		    },
    "mechanism_evidence": [
        {
						"pmid": "18247425",
						"description": "This is description for the mechanism evidence.",
						"evidence_types": [
								{
										"primary_type": "Function",
										"secondary_type": [
												"Biochemical"
										]
								},
								{
										"primary_type": "Rescue",
										"secondary_type": [
												"Human",
												"Patient Cells"
										]
								}
						]
				}
    ]
}