Allow customizing the Tanzu Hub endpoint when creating tanzu context

[anujc25]

What this PR does / why we need it

• Allow customizing the Tanzu Hub endpoint when creating a tanzu context.
• Implements a hidden flag tanzu-hub-endpoint with tanzu login and tanzu context create commands.
• If configured, the tanzu context will be created with the specified value in the tanzuHubEndpoint field under additionalMetadata.
• An additional log line This tanzu context is being created with the custom Tanzu Hub endpoint: "<specified-endpoint>" will be printed when this flag is specified while creating the context.

Which issue(s) this PR fixes

Fixes #

Describe testing done for PR

Verify with tanzu login

$ tz login --tanzu-hub-endpoint https://api.be-tis.symphony-dev.com/hub
[i] Opening the browser window to complete the login
Log in by visiting this link:

    https://console.cloud.vmware.com/csp/gateway/discovery?client_id=tanzu-cli-client-id&code_challenge=pVnmvp7umB7r0cGI-cvnU6yT8GVwj8vINhw_ASevFtM&code_challenge_method=S256&redirect_uri=http%3A%2F%2F127.0.0.1%3A52346%2Fcallback&response_type=code&state=ff12de5be3480b15c797115d05380dd1

    Optionally, paste your authorization code: [...]

[!] This tanzu context is being created with the custom Tanzu Hub endpoint: "https://api.be-tis.symphony-dev.com/hub"

[ok] Successfully logged into 'Tanzu Platform for K8s SaaS - Internal 1' organization and created a tanzu context

----

$ tz context get Tanzu_Platform_for_K8s_SaaS_-_Internal_1
name: Tanzu_Platform_for_K8s_SaaS_-_Internal_1
target: tanzu
contextType: tanzu
...
additionalMetadata:
    tanzuClusterGroupName: ""
    tanzuHubEndpoint: https://api.be-tis.symphony-dev.com/hub
    tanzuMissionControlEndpoint: https://tmc.tanzu.cloud.vmware.com
    tanzuOrgID: 8406e52e-6e36-445a-be7b-9dab6903341e
    tanzuOrgName: Tanzu Platform for K8s SaaS - Internal 1
    tanzuProjectID: 0f49c031-6294-4bdf-b363-55ae96788402
    tanzuProjectName: longevity-project
    tanzuSpaceName: tshaine

Verify with tanzu context create:

$ tz context create TP-context-with-custom-hub-endpoint --type tanzu --tanzu-hub-endpoint https://api.be-tis.symphony-dev.com/hub
? Enter control plane endpoint https://api.tanzu.cloud.vmware.com
[i] Opening the browser window to complete the login
Log in by visiting this link:

    https://console.cloud.vmware.com/csp/gateway/discovery?client_id=tanzu-cli-client-id&code_challenge=YDlk-X3c7vgZc98K_VdSb9-OxS_os2bNZA4ADc1GEKo&code_challenge_method=S256&redirect_uri=http%3A%2F%2F127.0.0.1%3A52981%2Fcallback&response_type=code&state=e0d2bf13bc93e612d2b5b77d6ac4dd7e

    Optionally, paste your authorization code: [...]

[!] This tanzu context is being created with the custom Tanzu Hub endpoint: "https://api.be-tis.symphony-dev.com/hub"

[ok] Successfully logged into 'Tanzu Platform for K8s SaaS - Internal 1' organization and created a tanzu context

---

$ tz context get TP-context-with-custom-hub-endpoint
name: TP-context-with-custom-hub-endpoint
target: tanzu
contextType: tanzu
...
additionalMetadata:
    tanzuHubEndpoint: https://api.be-tis.symphony-dev.com/hub
    tanzuMissionControlEndpoint: https://tmc.tanzu.cloud.vmware.com
    tanzuOrgID: 8406e52e-6e36-445a-be7b-9dab6903341e
    tanzuOrgName: Tanzu Platform for K8s SaaS - Internal 1

Release note

None

Additional information

Special notes for your reviewer

[vuil]

can we add a test case for this in context_test.go?