feat: show who an Azure account signs in as, in Details on its row (#5)

On an Azure account's group row, the Details pane now describes the account:

| field | from |
|---|---|
| name | token `name` |
| user (and "service principal") | az's account list |
| tenant: name · ID | az's account list (the name is borrowed from another entry in the same tenant when az left it out) |
| object ID | token `oid`; `y` on the row copies it |
| sign-in: MFA or not, and how | token `amr` |
| directory roles | token `wids`: built-in roles by name, others as `role 1a2b3c4d`, the default member role left out |
| subs: count · default | az's account list |
| cloud | az's account list, when not AzureCloud |

Nothing is fetched for it: the claims are the ones from the token lazysecrets already gets for the account's own tenant, read without verification. The token itself is never stored or shown. Before the first token, only the az-list fields show. A failing account shows its sign-in problem there instead (from #4).

New optional `provider.Accounts` interface; `azure.Provider` implements it. Tests: the fields from claims and az's list, sign-in methods, the UI showing it on the group row only, and `y` copying the object ID. Checked against a real signed-in account (name, MFA · password, default subscription) and an expired one (az-list fields only, since there's no token).

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---------

Co-authored-by: Karl-Einar Bergstrøm <karl-einarb@outlook.com>
Reviewed-on: #5

internal/provider/provider.go

@@ -184,3 +184,17 @@ type File struct {
 	Line int    // 1-based; 0 for the top
 	Sops string // the sops command to edit it with; empty for a plain file
 }
+
+// Accounts is implemented by providers that can describe the identity
+// they use, from what they have locally: for the Details pane on the
+// provider's row. ok is false when there's nothing to say.
+type Accounts interface {
+	Account() (Account, bool)
+}
+
+// Account describes a signed-in identity.
+type Account struct {
+	Fields []Attr
+	// Copy is what y copies on the provider's row, named CopyLabel.
+	Copy, CopyLabel string
+}