**Overview**:
- This PR integrates Odoo Sign with **emSigner**, a secure digital signing gateway
  that supports **Aadhaar-based eSignatures**.
- emSigner allows you to digitally sign a PDF by sending an HTTPS POST request
  with required data.
- Once the signing is complete, it returns the signed PDF in base64 format.

**purpose**:
- Connect Odoo with emSigner to enable digital document signing.

**Integration Steps**:

- Step 1: Generate unique Session Key for each request
- Step 2: Create JSON data and encrypt using unique Session key (Step 1)
- Step 3: Generate Hash of data
- Step 4: Encrypt generated Hash (Step3) with the unique Session Key (Step1)
- Step 5: Encrypt the unique Session Key (Step 1) using public key of the emSigner
- Step 6: HTTPS Post request with step2+step4+step5 output
- Step 7: Decrypt encrypted signed data (Step6) with session key (Step1)

**Workflow**:
- User will upload pdf on Odoo sign. user need to set auth method to emSigner.
- When the user clicks Validate& send button, they are redirected to emSigner for Aadhaar
  verification.
- The user enters their Aadhaar number, receives an OTP on their registered
  mobile, and completes verification.
- After successful Sign, emSigner redirects the user back to Odoo with the
  signed PDF.

**JSON data**:
- Here we have listed some of the important parameters.
- We need to send the PDF name, filetype(pdf), base64 data of the PDF, signature
  position where the signature will be added when user will sign. 
- You can refer to the documentation for a detailed explanation of the payload.

**Response from emSigner**:
- It will return a base64 data of pdf after digitally signing the pdf, and
  attaching digital certificate for signature integrity to all the signature
  positions we have provided in json data.

**Known limitation**:
- As emSigner takes only one pdf at a time, it will not work with bundle signing
  flow.
- After successful signing, emSigner will return pdf base64 data with the 
  digital signature attached, so writing anything after signing can break
  certificate integrity. 
- On emSigner other fields like name, email, company, selection etc is not
  supported, which is supported by odoo. As we need to write those values
  from odoo side which will break the certificate. 
- It allows having multiple signatories, we need to pass `isCosign=True`, 
  but again the issue will arise when writing odoo fields data on pdf will
  result in certification lose.
- Currently, due to this issue, we are allowing only one signer to use Aadhaar-based signing (via emSigner).
  Any other signers will use the regular signing method However, the regular signers must complete 
  their signing before the Aadhaar signer can sign.
- To make this work, we have set a default signing order whenever an Aadhaar (emSigner) method is used.
- When using eMSigner, the final signed document will not include the certificate from Odoo. 

IAP PR: https://github.com/odoo/iap-apps/pull/1009

**Emsigner Documentation** : https://developers.emsigner.com/signer-gateway/api-reference/signing-documents.html

task-4164342