feat: fix routines as templates with the caller's values; deprecate run_device_action (4.13.0)

run_device_action was a string dispatch drivers filled with an
application's remediations, values hard-coded inside (community
"public", the caller's address, a /24 firewall rule).

napalm_device_types.fixes declares them as templates instead. A
template runs declared steps in order and stops at the first failure
(run_fix_steps); the driver implements the steps, or overrides the
whole method where its device takes the change in one call; every
value comes from the caller.

- SnmpAgentMixin.configure_snmp_agent(settings): install, configure,
  firewall, start. The caller verifies with its own SNMP client.
  get_management_peer() (optional) says the address the device sees the
  management connection come from, for choosing allowed_sources.
- EnsurePackageMixin.ensure_package(name): install only what is missing.
- PackageProxyMixin.configure_package_proxy(url): declared.
- ContainerEngineMixin.grant_container_engine_access(user): declared.

None is mixed into a role base, so a capability check stays truthful.

Closes #33

README.md

@@ -91,6 +91,15 @@ is a thin bundle over them — `PackageManagementMixin`, `HealthMetricsMixin`,
 `HostRebootMixin` (`reboot_host`) is mixed into `DeviceTypeDriver` itself, since any
 device may be restartable; like the others it only declares.
 
+**Fixes are templates** (`napalm_device_types.fixes`, since 4.13). A change the
+application wants made on a device runs as fixed steps the driver implements, with values
+the caller supplies: `SnmpAgentMixin.configure_snmp_agent(settings)` (install, configure,
+firewall, start), `EnsurePackageMixin.ensure_package(name)`,
+`PackageProxyMixin.configure_package_proxy(url)` and
+`ContainerEngineMixin.grant_container_engine_access(user)`. A driver whose device takes the
+change in one call overrides the template method. A driver mixes in the fixes its device
+can take; none is on a role base. They replace `run_device_action`, which is deprecated.
+
 **SNMP is the application's** (`docs/STRUCTURE.md`). `HealthMetricsMixin` is deprecated
 since 4.12 and goes in 5.0. A driver does not collect over SNMP; a driver whose device
 publishes its health values under OIDs of its own declares them as data: