extsvc: rename `AccountSpec` to `Spec`

Created by: unknwon

To be consistent on field and type naming.

It is not an ideal name if we only look at the type alone, but makes a lot more sense when we actually use the field, e.g.

var acct extsvc.Account
acct.Spec = xyz
// Not acct.AccountSpec which feels redundant

Created by: codecov[bot]

Codecov Report

Merging #9545 into master will not change coverage. The diff coverage is 90%.

@@           Coverage Diff           @@
##           master    #9545   +/-   ##
=======================================
  Coverage   41.49%   41.49%           
=======================================
  Files        1334     1334           
  Lines       72684    72684           
  Branches     6583     6583           
=======================================
  Hits        30159    30159           
  Misses      39723    39723           
  Partials     2802     2802

Impacted Files	Coverage Δ
internal/extsvc/types.go	0% <ø> (ø)
cmd/frontend/authz/perms.go	30% <ø> (ø)
cmd/frontend/auth/user.go	76.81% <ø> (ø)
...prise/cmd/frontend/internal/authz/github/github.go	58.53% <0%> (ø)
...nterprise/cmd/frontend/auth/gitlaboauth/session.go	0% <0%> (ø)
cmd/frontend/db/repos_perm.go	75.4% <100%> (ø)
...erprise/cmd/frontend/internal/authz/gitlab/sudo.go	81.13% <100%> (ø)
...rontend/internal/authz/bitbucketserver/provider.go	79.06% <100%> (ø)
.../cmd/frontend/internal/authz/gitlab/gitlab_sudo.go	72.72% <100%> (ø)
enterprise/cmd/frontend/auth/openidconnect/user.go	74.46% <100%> (ø)
... and 7 more