feat: Social Logins Implementation

[PaRaDoX50]

Description

Added Google Sign In to the app. Was not able to implement Apple Sign In as it requires setting up a paid apple developers account.
This change depends on PR 719 of mentorship-backend which is not merged yet.Therefore, merged PR 719's code with the forked mentorship-backend repo and used it as backend.

For setting up google sign in for android:
1.) Create a google console project.
2.) Create OAuth client id for web app(Use this client id in backend) and android(by putting in correct application id and sha1 code).
3.) Under res/values/strings.xml put

 <?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="default_web_client_id">{Web app Client id goes here}</string>
</resources>

(As you need a backend client id for extracting idToken after google sign in, refer documentation) .
4.) Make sure you use the same web app client id in backend.

For iOS:
I have not tested the code in iOS as a mac is not available to me.

Fixes #137 (Not completely)

Flutter Channel:

• I have used the Flutter Beta channel on my local machine

Type of Change:

• Code

Code/Quality Assurance Only

• This change requires a documentation update (software upgrade on readme file)
• New feature (non-breaking change which adds functionality pre-approved by mentors)

How Has This Been Tested?

Sign In:

Logout:

Google Sign In works as it should but breaks change password feature.

Checklist:

Delete irrelevant options.

• My PR follows the style guidelines of this project
• I have performed a self-review of my own code or materials
• I have commented my code or provided relevant documentation, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• Any dependent changes have been merged

Code/Quality Assurance Only

• My changes generate no new warnings
• My PR currently breaks something (fix or feature that would cause existing functionality to not work as expected)
• Any dependent changes have been published in downstream modules

[techno-disaster]

@yugantarjain does the password feature break on iOS too? Any workaround?

[techno-disaster]

@yugantarjain does the password feature break on iOS too? Any workaround?

@anitab-org/mentorship-ios-maintainers Could anyone of you confirm if this was the case on ios too? If yes any workarounds?

Thanks!